The present invention relates to phase shift circuitry, and in particular, to passive phase shift circuitry providing a substantially constant phase shift over a wide frequency band.
Many of today's electronic devices use wireless signal technologies for both connectivity and communications purposes. Because wireless devices transmit and receive electromagnetic energy, and because two or more wireless devices have the potential of interfering with the operations of one another by virtue of their signal frequencies and power spectral densities, these devices and their wireless signal technologies must adhere to various wireless signal technology standard specifications.
When designing such wireless devices, engineers take extra care to ensure that such devices will meet or exceed each of their included wireless signal technology prescribed standard-based specifications. Furthermore, when these devices are later being manufactured in quantity, they are tested to ensure that manufacturing defects will not cause improper operation, including their adherence to the included wireless signal technology standard-based specifications.
When testing radio frequency (RF) devices and systems in general, and wireless RF devices and systems in particular, there is often a need for shifting the phase of a signal being transmitted or received via a particular signal path. For example, when testing devices using one or more wireless signal paths, such as within a shielded enclosure or another form of controlled signal path environment, one or more antenna elements (e.g., an antenna array) may be used along with phase shifting elements to allow for shifting of signal phases within the one or more signal paths between the signal source and each antenna element so as to mitigate multipath signal interference effects. (Such test enclosures and wireless signal testing techniques are disclosed in U.S. Patent Publications 2014/0266929 and 2014/0266930, the contents of which are incorporated herein by reference.)
A variety of RF signal path structures exist that can produce variable amounts of phase shift. For example, simply having two transmission lines of different lengths will cause the signals conveyed by such lines to experience mutually distinct phase shifts, thereby causing a phase shift of one signal relative to the other. However, simply using a selected length of transmission line will introduce a phase shift that varies as a linear function of signal frequency. Accordingly, a desired amount of phase shift can only be achieved over a very narrow bandwidth.
One technique that has been developed to increase the bandwidth available over a passive transmission line is known as the Schiffman phase shifter design, which uses a transmission line and a coupled section to provide a wider bandwidth over which a desired phase shift can be imparted. However, achieving that wider bandwidth requires tight signal coupling between transmission line elements, which can make implementation difficult.
Another technique that has been developed, often referred to as a compact ultra wideband phase shifter, can achieve a wide phase shift bandwidth (e.g., 3-11 GHz). However, the phase difference is limited to 30 degrees or less.
Accordingly, it would desirable to have a technique for providing selectable amounts of significant phase shift, e.g., 90 degrees or more, over a wide frequency band.